This invention relates to pressure transmitters and fluid isolators for pressure transmitters.
Industrial pressure transmitters typically include an isolator for isolating a pressure sensing diaphragm in the transmitter from chemical attack by process fluids. Pressurized process fluid deflects an isolator diaphragm which pressurizes an isolator fluid such as silicone oil which deflects the sensing diaphragm. It is desirable to have the isolator diaphragm thin, flexible and compliant so that there is only a small pressure drop across it. On the other hand, it is desired to have at least a minimum thickness isolator diaphragm to avoid rupture of the isolator diaphragm with normal use under field conditions. This thickness of the isolator diaphragm is a compromise between conflicting requirements for compliance or deflectability and ruggedness.
Pressure drops across isolator diaphragms introduce errors in the pressure sensor output. Errors which are repeatable-type errors can be corrected for in calibration by linearization and temperature correction algorithms stored in a microcomputer in the transmitter which controls the transmitter output. Non-repeatable errors due to non-elastic deformation of the isolator diaphragm such as temperature hysteresis and line pressure hysteresis tend to be unpredictable and can't be simply corrected by algorithms. These unpredictable errors limit the performance of the transmitter, particularly its long term drift characteristics. Unpredictable errors tend to increase with high stress experienced by the isolator diaphragm, and it is thus desired to arrange an isolator in a transmitter which has a reduced amount of stress in the isolator diaphragm to reduce non-repeatable errors and long term drift.